
Models in which \(U(1)_1=U(1)_Y\) . The kinetic mixing occurs before the SSB. Our model and the one of Ref. [1]} are of this type.

Models in which \(U(1)_2=U(1)_Q\) , i.e., the kinetic mixing occurs after the SSB and the mixing is with the massless photon. An example of this type of model is that of Holdom in [2]}.

Neither \(U(1)_1\)  nor \(U(1)_2\)  are related to the symmetry of the SM directly, but \(U(1)_1\otimes U(1)_2\rightarrow U(1)_Y\)  after SSB. The Galison and Manohar model [2]} is of this type.

